I think the 175 is as good a plane as the 172 or maybe even the 182. However, if it has the geared O-300, it may be more difficault to resell since many shy away from that engine I think the engine is fine... I doubt if anyone can tell you WHY they don't want the O-300).
Airplane prices are down right now... $20k is not a bad deal, but it is an average deal. I see flying 172's going for as little as $14K now.
Without knowing how to handle a geared engine, the geared 0-300 can be a maintenance problem, and there are no new parts except cylinders. Not a lot of those engines used compared to the direct drive 0-300s, even those are becoming rare on the used market. The plus is the cabin is the same size as the 182; the load is in the middle of the 172 and 182. There is STCs if you just love your 175 to put 0-360's and 0-470s on the nose making it more 182 like. I dont think I have seen a 172 in equal shape as a 175 that was cheaper than a 175.